1. Field of the Invention
The present invention relates to an image processing apparatus, method and program that suppresses a spatial frequency component corresponding a periodic pattern in an image signal. In particular, the present invention relates to an image processing apparatus, method and program that suppresses a periodic pattern in a radiographic image caused by a grid used in radiography.
2. Description of the Related Art
Conventionally, storable phosphors (photostimulable phosphors) have been used. When a storable phosphor is irradiated with radiation (X-rays, α-rays, β-rays, γ-rays, an electron beam, ultraviolet rays or the like), apart of radiation energy is stored in the storable phosphor. After then, when excitation light, such as visible light and a laser beam, is output to the storable phosphor, the storable phosphor emits photoluminescence corresponding to the radiation energy stored therein. For example, a radiographic image readout apparatus using the storable phosphor is widely used in CR (Computed Radiography). A storable phosphor sheet, in which the storable phosphor is deposited on a substrate, is irradiated with radiation that has passed through a subject, such as a human body, and radiographic information is temporarily stored and recorded on the storable phosphor sheet. Excitation light, such as a laser beam, is output to the storable phosphor sheet to induce photoluminescence. Further, photoelectric conversion is performed on the photoluminescence to obtain an image signal.
Here, when a radiographic image of a subject is imaged and recorded on the storable phosphor sheet or the like, imaging is performed by placing a grid between the subject and the sheet in some cases so that radiation scattered by the subject does not irradiate the sheet. In the grid, lead or the like, which does not pass radiation therethrough, and aluminum, wood or the like, which tends to pass radiation therethrough, are alternately arranged at a narrow pitch of 4 line/mm. When radiography is performed by using the grid, radiation scattered by the subject does not tend to irradiate the sheet. Therefore, it is possible to improve the contrast of the radiographic image of the subject. However, when the size of the image including this grid image is enlarged or reduced, aliasing due to folding occurs depending on the magnification or reduction ratio. Further, when aliasing overlaps with the spatial frequency of the grid image or the like, a narrow stripe pattern (moire) is generated, and observation of a regenerated image becomes difficult. Therefore, there is a demand for a method for detecting the direction of a periodic pattern, such as a grid image and a moire, and a spatial frequency component corresponding to the periodic pattern, and removing the spatial frequency component corresponding to the periodic pattern in the detected direction.
U.S. Pat. No. 5,661,818 (Patent Document 1) discloses, as a method for removing such a periodic pattern from an image, a method in which filtering is performed in two directions of a vertical direction and a horizontal direction of the image by a high-pass filter, and variance values of the result of filtering are calculated with respect to the two directions of the vertical direction and the horizontal direction, and a direction in which a grid image is present is detected based on the ratio of the variance values.
Further, Japanese Unexamined Patent Publication No. 2003-233818 (Patent Document 2) discloses a method in which peaks of frequency patterns are detected with respect to a horizontal direction and a vertical direction. When the peaks of the frequency patterns have been detected in both of the horizontal direction and the vertical direction, a direction in which the magnitude of the peak is greater than a predetermined threshold is judged as the direction of a grid image.
Further, U.S. Pat. No. 8,097,839 (Patent Document 3) discloses an image processing method. In the method, two-dimensional Fourier transformation is performed on image data to detect a spatial frequency component corresponding to a periodic pattern. Further, filtering is performed by using a filter that removes only the detected spatial frequency component.